1. Field of the Invention
The present invention relates to a radar device for detecting a target by using a radio wave.
2. Description of the Related Art
An FM-CW radar system is known as a radar system which is installed on a moving object, such as a vehicle, to detect a target, such as another vehicle, a human being, or an obstacle.
The FM-CW radar transmits a frequency-modulated continuous transmission signal and receives a reflected signal from a target. The received signal and the transmission signal are mixed to generate a beat signal thereof. The relative speed and the relative distance of the target are then determined from the beat signal.
The frequency of the beat signal is determined as follows. First, a window function is applied to the beat signal, and the resultant signal is subjected to a FFT process to determine the frequency spectrum of the beat signal. FIG. 8A shows an example of a beat signal in the time domain, and FIG. 8B shows an example of a signal in the time domain which is obtained by applying the window function to the original beat signal. FIG. 8C shows the frequency spectrum of the signal shown in FIG. 8B. In the frequency spectrum, a portion having increased signal strength (power) includes main frequency components of the beat signal. The frequency of the peak of that portion corresponds to the frequency of the beat signal (hereinafter, such a frequency will be referred to simply as a peak frequency).
However, the spectrum obtained via the discrete Fourier transform, such as FFT, is a discrete spectrum including components at frequency intervals equal to 1/T where T is the sampling interval. This means that, in the detection of the peak frequency of the beat signal, the highest possible frequency resolution is determined by the frequency interval (FFT range bin) equal to 1/T. Hereinafter, the frequency of a discrete frequency spectrum component having the highest signal strength will be referred to as the discrete peak frequency.
In such a radar device, techniques which improve the peak frequency resolution are disclosed in the following patent related documents: (1) Japanese Patent No. 3213143; (2) Japanese Unexamined Patent Application Publication No. 2001-42033; and (3) Japanese Unexamined Patent Application Publication No. 10-213613.
In the radar device disclosed in (1), the barycenter of the signal strength (power) is determined from data indicating a discrete frequency spectrum, and the barycentric frequency is used as the peak frequency. FIGS. 9A to 9C show a process of determining the peak frequency according to the above-identified method. More specifically, FIG. 9A shows an example of a beat signal in the time domain, and FIG. 9B shows an example of a signal in the time domain, obtained by applying a window function to the original beat signal shown in FIG. 9A. FIG. 9C shows the frequency spectrum of the signal shown in FIG. 9B. As shown FIG. 9C, the frequency at the barycenter of a portion including increased spectrum components is detected as the peak frequency.
In the radar device disclosed in (2), after applying a window function to a beat signal, the amount of data is increased by adding data with an amplitude of zero, and the result is subjected to a discrete Fourier transform. FIGS. 10A to 10C show a process according to the above-identified method. More specifically, FIG. 10A shows an example of a beat signal in the time domain, and FIG. 10B shows a signal in the time domain, obtained by adding data having an amplitude of zero after applying a window function to the beat signal shown in FIG. 10A. FIG. 10C shows the frequency spectrum of the signal shown in FIG. 10B. As shown in FIG. 10C, the resultant spectrum has a greater number of components obtained by interpolation, such that the peak frequency is detected with a higher detection resolution.
In the radar device disclosed in (3), the difference between the frequency of a true peak point and the frequency of a discrete frequency spectrum component having the greatest signal strength is determined from the ratios of the components having the greatest signal strength to the signal strength of higher and lower spectrum components adjacent to the component having the greatest signal strength.
However, in radar devices using any of the techniques disclosed in (1), (2), or (3), a great amount of computation is needed to detect the true peak frequency. That is, to determine the peak frequency with high accuracy, sampling must be performed at very short intervals, and thus, a very high performance processor is needed, which results in greatly increased circuit complexity and cost. Although the peak frequency can be determined with high accuracy using a low-performance processor, it takes a long time to calculate the peak frequency, and thus, it is impossible to detect a target within a sufficiently short time.
To overcome the above-described problems, preferred embodiments of the present invention provide radar device which detects a target with high accuracy by detecting a true peak frequency with high accuracy via a calculation which requires a greatly reduced amount of computation.
Preferred embodiments of the present invention provide a radar device for detecting a target from a peak frequency of a beat signal, including a transmission unit for transmitting a frequency-modulated transmission signal and generating a beat signal between a portion of the transmission signal and a signal reflected from the target, a signal processing unit for determining a discrete frequency spectrum for the beat signal multiplied by a window function and for determining the peak frequency of the beat signal from the discrete frequency spectrum, wherein the true peak frequency of the beat signal is determined by performing a process including the steps of fitting the frequency spectrum of the window function to the discrete frequency spectrum of the beat signal, determining the peak frequency of the window function fit to the discrete frequency spectrum of the beat signal, and using the peak frequency of the window function as the true peak frequency of the beat signal.
In the radar device according to preferred embodiments of the present invention, the true peak frequency of the beat signal is determined on the basis of an expression indicating xcex94f as a function of signal strength at plural discrete frequencies in the discrete frequency spectrum, where xcex94f is the frequency difference between the peak frequency of the frequency spectrum of the window function and the discrete peak frequency in the discrete frequency spectrum of the beat signal.
Furthermore, in the radar device according to preferred embodiments of the present invention, the function indicates the relationship between the frequency difference xcex94f and the ratio xcex94p between values of signal strength at two discrete frequencies of the plural discrete frequencies.
Furthermore, in the radar device according to preferred embodiments of the present invention, the two discrete frequencies are discrete frequencies in the discrete frequency spectrum, which are respectively higher and lower than the peak frequency and which are adjacent to the peak frequency in the discrete frequency spectrum.
Furthermore, in the radar device according to preferred embodiments of the present invention, the window function is preferably a function of a Hanning window, and the frequency difference xcex94f is determined on the basis of a linear function approximating the function indicating the relationship between the frequency difference xcex94f and the ratio xcex94p in logarithm between the values of signal strength at the two discrete frequencies.
In the radar device according to preferred embodiments of the present invention, the true peak frequency of the beat signal is determined by performing a process including the steps of fitting the frequency spectrum of the window function to the discrete frequency spectrum of the beat signal, determining the peak frequency of the window function fit to the discrete frequency spectrum of the beat signal, and using the peak frequency of the window function as the true peak frequency of the beat signal. Thus, the true peak frequency of the beat signal with a higher frequency resolution than the frequency resolution of the discrete frequency spectrum of the beat signal is easily determined.
Furthermore, in the radar device according to preferred embodiments of the present invention, an expression indicating xcex94f as a function of signal strength at plural discrete frequencies in the discrete frequency spectrum is determined in advance, where xcex94f denotes the frequency difference between the peak frequency of the frequency spectrum of the window function and the discrete peak frequency in the discrete frequency spectrum of the beat signal, and the true peak frequency of the beat signal is easily determined via a simple calculation using the function.
Furthermore, in the radar device according to preferred embodiments of the present invention, by using the function indicating the relationship between the frequency difference xcex94f and the ratio between values of signal strength at two discrete frequencies of the plural discrete frequencies, it is possible to determine the true peak frequency of the beat signal via a calculation which requires a greatly reduced amount of computation.
Furthermore, in the radar device according to preferred embodiments of the present invention, if discrete frequencies in the discrete frequency spectrum, which are respectively higher and lower than the peak frequency and which are adjacent to the peak frequency in the discrete frequency spectrum, are used as the two discrete frequencies described above, the frequency difference xcex94f is very sensitive to changes in the ratio xcex94p between the values of signal strength at the two discrete frequencies, and thus, the true peak frequency of the beat signal is determined with higher accuracy via a simple calculation.
Furthermore, in the radar device according to preferred embodiments of the present invention, when a Hanning window is used as the window function, and the relationship between the frequency difference xcex94f and the signal strength ratio xcex94p is approximated by a linear function, and the true peak frequency of the beat signal is determined via a further simplified calculation.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.